Conversions: Use new CSS classes

These new CSS classes allow Goodies to work well with themes.
Update example image in README
Merge pull request #511 from duckduckgo/jag/conversion-new-css

Conversions: Use new CSS classes
Calculator: use proper quoting of regex chars.

The old version clearly worked, but it was clunky and somewhat
inexplciable.  While reading up on something else, I discovered `perldoc
quotemeta` which resolves this in a much more predictable and correct
way.
Calculator: refactor the output preprocessing.

I think this makes it somewhat easier to follow the intent here.  We're
just trying to convert these two known strings into different output
formats, so wrapping that up seems like a good idea.
Calculator: superscripting for arbitrary terms.

This would be better with some proper parsing, but this resolves some
issues with the current HTML output, making it raise things to
arbitrarily high superscripting levels based on what it seems to look
like.
NumberStyler: add proof-of-concept

Considering how all of this works, I'm not sure roles are quite the
right fit. It seems like we might just want a shared "Utility" package
or namespace to let us have access to some common library functions.

DDG::GoodieRole::NumberStyle is a particularly horrible name because
it's not even a role!

Also, the comments suck.

But such is the nature of a proof-of-concept.
NumberStyle: minimal changes for Conversions.

I am not claiming this is entirely the correct way to do this in this
package, just that it works fairly easily to expand the scope of the
Conversions goodie, as well.

Gonna need feedback from @moollaza and @mintsoft.

On the upside, I found a bug in the styler code by writing a test here!